For several decades in the field of corrosion resistant metals most inventive effort has been directed toward development of nickel base or low iron content alloys. While relatively little effort has been made to improve austenitic iron base alloys of low special element content, their importance may be judged by the fact that the ordinary 18% Cr--8% Ni family of stainless steels is employed more than all other corrosion resistant alloys combined. However, those steels have very limited utility in handling sulfuric acid solutions.
Because of cost, availability and metallurgical factors, alloys intended to resist sulfuric acid contain several or all of the elements from the group, iron, nickel, chromium, molybdenum, copper, manganese, silicon and columbium. The nominal compositions of commercial alloys intended for sulfuric acid service as well as for many other applications are shown in Table I. The carbon content is approximate.
TABLE I ______________________________________ Alloy Designation Ni Cr Mo Cu Cb Si Fe C ______________________________________ 20 29 20 2.5 3.5 -- 1. 47 .15 20Cb3 34 20 2.5 3.3 .6 .6 38 .06 20Mo4 36 23 4.0 1.0 .25 .4 35 .02 20Mo6 35 24 6.0 3.0 -- .2 30 .02 825 42 22 3.5 2.5 -- .6 26 .03 G-3 45 22 7.0 2.0 .8 .6 22 .01 625 63 22 9.0 - 4.0 .6 1 .06 ______________________________________
While carbon contents above about 0.08% do not reduce the sulfuric acid resistance of these alloys, at very low carbon levels, less than about 0.03%, they have been found to have good resistance to a wide spectrum of other corrosive substances.
Parson, U.S. Pat. No. 2,185,987, discloses the first notable alloy of this group, commonly referred to as Alloy 20.
To avoid intergranular attack in weld zones, columbium was added to Alloy 20. Because Alloy 20 was reported to suffer stress corrosion in certain concentrations of sulfuric acid, Scharfstein, U.S. Pat. No. 3,168,397, developed the higher nickel 20Cb3 alloy, which was claimed to overcome this problem. DeBold, et al., U.S. Pat. No. 4,487,744, discloses the still higher nickel content 20Mo4 alloy, which is claimed to provide improved resistance to many chemical substances including sulfuric acid. Later, Henthorme, et al., U.S. Pat. No. 4,201,575, developed alloy 20Mo6, which was claimed to have good general corrosion resistance as well as resistance to chloride solution pitting and crevice corrosion. Alloys 825 and G-3 have even higher nickel and lower iron contents, while Alloy 625 has virtually eliminated iron. While these high nickel alloys provide remarkable resistance to many other corrosive substances, their resistance to sulfuric acid solutions at ambient temperatures is achieved at very high cost and consumption of the nonferrous elements involved. Also, while all of these alloys are furnished in wrought or cast forms, fabricability and weldability tend to be lower as iron content drops. Other similar high nickel alloys not listed above possess even lower fabricability and weldability. Thus, there has remained a need for leaner, rather than richer, principally Ni but also Cr containing alloys for the handling of sulfuric acid at ambient temperatures which are also very ductile and weldable.